CLAYTON — A wildfire burning in a San Francisco Bay Area wilderness park grew Monday, forcing more people to leave their homes and leading to a smoke advisory for area residents.

The blaze in Mount Diablo State Park in Contra Costa County spread to 3,718 acres or nearly 6 square miles Monday afternoon, more than double the 1,500 acres reported in the morning, according to the California Department of Forestry and Fire Protection. It was 20 percent contained.

The flames were threatening 100 homes in the sparsely populated area, which is dotted with animal pens and shooting ranges. The fire was also threatening electrical transmission lines, communications infrastructure, and a historic lookout and visitor center at the top of the 3,848-foot Mount Diablo. The Summit Museum was constructed in the 1930s of sandstone from the park.

The blaze broke out Sunday amid nearly triple-digit temperatures in the early afternoon. The cause was under investigation.

The fire spewed a plume of smoke visible for miles. It was burning in steep, rugged terrain near Clayton, a town of about 11,000 people northeast of San Francisco, alongside the park.

In addition to the difficult terrain, firefighters faced erratic winds and continued high temperatures. But they were confident they would get the blaze under control, Cal Fire spokesman David Shew said.

“We’ll get it, but it will probably be a few days,” he said.

More than 700 firefighters, aided by two air tankers and three water-dropping helicopters, were battling the blaze.

The Bay Area Air Quality Management District issued a smoke advisory Monday for parts of Contra Costa, Alameda and Santa Clara counties. Residents were advised to take precautions, including setting air conditioning units and car vent systems to recirculate.

Elderly people, children and those with respiratory illnesses were told to be particularly careful.